The present invention relates to a laser disk player and, more particularly, to a synchronization holding circuit used in scanning within a laser disk player which holds coincidentally the synchronization of each line during a scan mode. The laser disk player is classified as a constant linear velocity (hereinafter, referred to as CLV) system and a constant angular velocity (hereinafter, referred to as CAV) system according to methods of recording video and audio information onto a laser disk.
In the CAV system, the same amount of video and audio information are recorded on inner and outer tracks and the speed of revolution is maintained at 1800 rpm at both innermost and outermost tracks when the video and audio information is detected by an optical pickup. In the CAV system, since a horizontal synchronizing signal and a color burst synchronizing signal are arranged coaxially the horizontal synchronization agrees with the color burst synchronization and advantageously images are not destroyed even in the scan mode. But, both of the innermost and the outermost tracks should be recorded with the same amount of information, thereby resulting in wasted disk space.
The CLV system has been introduced to solve such problems. In the CLV system, the amount of information recorded in the outermost track is as much as three times more than that in the innermost track. The innermost track revolves at a speed of 1800 rpm, while the outermost one revolves at a speed of 600 rpm when the information stored on the laser disk is detected by the optical pickup.
FIG. 1 shows a circuit diagram of a conventional laser disk player employing the conventional CLV system. In FIG. 1, if a user presses a playback key (not shown in FIG. 1) for initially driving of a laser disk 1, a MICOM (microcomputer) 2 supplies power to a spindle motor 3 and a pickup driving servo system 4 for rotating a turntable 5 and locates an optical pickup 6 in the inner or outer track of the laser disk 1.
The location of the optical pickup 6 is determined by the starting point of the information, where a frequency generating sensor 7 detects rotational speed of the spindle motor 3 and provides such a sensed signal to a time-based controller 8. The time-based controller 8 recognizes the current rotational speed of the spindle motor 3 by this sensed signal and controls a spindle motor servo system 9 to maintain normal speed of the spindle motor 3. The time-based controller 8 is controlled by the sensed signal which is supplied from the frequency generation sensor 7 only during the initial driving of the spindle motor 3, while the time-based controller 8 is controlled by another signal which is supplied from an adder 16 in the next step if the speed of the spindle motor 3 is beyond a predetermined value.
Particularly, the spindle motor 3 revolves at the speed of 1800 rpm when the optical pickup 6 detects the information stored on the inner track of the laser disk 1, while revolving at the speed of 600 rpm when the optical pickup 6 detects the information stored on the outer track of the laser disk 1 so as to detect the information stored on the laser disk 1 safely. Next, audio and video signals detected by the optical pickup 6 are respectively provided to an audio processor 10 and a video processor 11, and they are applied to a speaker and a monitor (which are not shown in FIG. 1) after signal processing in the audio processor 10 and the video processor 11.
Subsequently, a horizontal synchronizing signal detector 12 separates a horizontal synchronizing signal out of the video signal which comes from the video processor 11, while a color burst synchronizing signal detector 13 separates a color burst synchronizing signal in the burst signal interval of the video signal. Then, the horizontal synchronizing signal is compared with a reference horizontal synchronizing signal of 15.734 KHz in a horizontal synchronizing error detector 14. The color burst synchronizing signal is also compared with a reference color burst synchronizing signal of 3.58 MHz in a color burst synchronizing error detector 15. The error signals which are provided from the horizontal and color burst synchronizing error detectors 14 and 15 are added by the adder 16 and the output of the adder 16 is provided to the time-based controller 8. Then, the time-based controller 8 controls the speed of the spindle motor 3 in response to the summed error signal which is provided from the adder 16. Thus, the spindle motor 3 can maintain the constant speed, so that the optical pickup 6 can accurately detect the information stored on the laser disk 1.
In this CLV system, if a user presses scan switches SCAN+ or SCAN- of the MICOM 2 for scan mode, the MICOM 2 not only rotates the spindle motor 3 at a constant speed, but also radially moves the optical pickup 6 to the inner or outer side of the laser disk 1 to execute the scan mode. In the CLV system, however, the horizontal synchronizing signals are not arranged in one line on the laser disk 1 since the amount of the information on the outer side of the laser disk 1 is as much as about three times more than that on the inner side.
In the scan mode, therefore, the synchronization of video signals to be displayed on a screen do not agree with each other, so the video signal is not displayed on the screen. In order to solve this problem, random access memories are employed. In other words, the random access memory, first, stores the information corresponding to one field of the video signal and next applies the information to the monitor to prevent the non-displaying mode on the screen. But, this method is not practical since the cost of the random access memory is very expensive. In addition, images on the screen are not clear since the video signal corresponding to only one field is applied to the monitor.